Electric and hybrid powered vehicles (EV/HEV) may include a higher voltage bus, such as a 42 V bus. Integrated starter/alternators (ISA) have been used in low storage requirement applications such as light hybrid vehicles. In most ISA systems, a standard six-switch inverter topology that is illustrated in FIG. 1 is used.
The inverter 50 utilizes six switches 52 that selectively block the battery voltage Vb of 42 V (=1 pu) and carry the rated peak phase current of the motor 54 (=1 pu). Employing space vector pulse width modulation (PWM), the inverter 50 produces a peak motor phase voltage of 0.578 pu in the linear region and 0.637 pu in a six-step mode when the motor is Y-connected. Although the Y-connection is illustrated in FIG. 1, the inverter 50 can also be used with a delta connection.
A useful measure of topology cost of the inverter is the switch utilization ratio (SUR). The SUR relates the required kVA of the silicon switches to the output volt-amperes that can be produced for a unity power factor load. Mathematically, this is:
                              (          SUR          )                =                                            (              VA              )                                      3              ⁢                              -                            ⁢              phase                                                          (                              number                ⁢                                                                  ⁢                of                ⁢                                                                  ⁢                switches                            )                        ⁢                          V              SW                        ⁢                          I              SW                                                          (        1        )            where VSW and ISW are the switch voltage and current ratings. The inverter 50 of FIG. 1 has an SUR of 0.159.